This invention relates to heating elements made of silicon carbide and more particularly relates to a method for making a recrystallized silicon carbide resistor having siliconized silicon carbide contacts.
Methods for making recrystallized silicon carbide resistors having siliconized silicon carbide contacts have been previously known. Such methods have involved the fusion of siliconized silicon carbide contacts with a recrystallized silicon carbide resistor by holding the resistor and contacts together, e.g. with a bonding resin, and indirectly heating the combination in a gas or electrical resistance heated furnace, e.g. a tube furnace, to fuse the contacts to the resistor by flow of silicon from the contacts into the resistor.
Such known methods are replete with problems and disadvantages. Such tube furnaces are typically made of graphite in order to withstand the high temperatures required, e.g. in excess of 1800° C. The graphite tube of the furnace is expensive and is consumed during repeated heating and cooling cycles. In addition to cost of the graphite tube, replacement causes costly and inefficient down time.
Further, in resistance heated furnaces, relatively costly graphite electrodes must be periodically replaced again resulting in costly down time.
Other consumable components for graphite tube furnaces include graphite crucibles or boats that carry parts, insulation, and various electrical components including transformers.
Fusion of contacts to resistors, as described above, require vacuum or an inert atmosphere. Due to cooling requirements for the shell of the furnaces, the furnaces must be large relative to parts being treated, thus use of a vacuum is impractical and the required volume of inert gas is very large relative to the size of the parts thus adding more inefficiency and cost.
Additionally, such known methods are exceedingly inefficient with respect to required heat energy. As with most indirect heating methods, energy is lost by undesirably heating furnace components rather than the article to be treated. In particular, there are heat losses in the furnace wall, loss through radiation, loss through conveyors for parts, atmospheric heat losses and undesirable heat storage. Further, in known methods, the entire part was heated rather than only a localized area of the part that required heating. The efficiency in heating the contacts to fuse them to the resistor in known method could thus be expected to be less than 5%.